The interactions between the antitumor-active metallocene molybdocene dichloride (Cp2MoCl2) and four oligonucleotides have been studied by H-1 and P-31 NMR spectroscopy. In 50 mM salt solutions of molybdocene dichloride, hydrolysis of the halide ligands occurs to give a solution with pD 2, containing a species in which both Cp rings remain metal bound for 24 h. At pD 7, however, partial hydrolysis of the Cp rings (similar to 30%) occurs after 24 h. Addition of an aqueous solution of molybdocene dichloride in 50 mM salt to the self-complementary sequence d(CGCATATGCG)(2), maintaining the pD at 6.0-7.0, showed no evidence for the formation of a metallocene-oligonucleotide complex, and only peaks arising from hydrolysis of molybdocene dichloride were detected. A similar result was obtained in titration experiments with the single-stranded sequence d(ATGGTA) at pD 6.5-7.0. However, at pD 3.0, new signals assigned to a molybdocene-oligonucleotide complex or complexes were detected in the H-1 NMR spectrum. No change was observed in the P-31 NMR spectrum. The complex or complexes formed between molybdocene dichloride and d(ATGGTA) are stable for hours at pD 3.0; at higher pD, the complex is destabilized and only peaks arising from hydrolysis of molybdocene dichloride are detected. Titration experiments at low pD with the dinucleotide dCpG showed a new set of signals in the H-1 NMR spectrum, tentatively assigned to formation of a complex arising due to coordination of molybdenum to guanine N7 and/or cytosine N3. At pD 7.0, these signals disappeared. The results obtained show chat stable oligonucleotide adducts are not formed in 50 mM salt at PD 7.0, and hence it is highly unlikely that formation of molybdocene-DNA adducts in vivo is the primary action that is responsible for the antitumor properties of molybdocene dichloride.